The present invention relates to a novel method of reducing radioactivity, particularly to a method of suppressing adherence of radioactive substances to structural materials used in contact with a liquid containing the radioactive substances dissolved therein, for example, primary cooling water piping in a nuclear power plant, and to a method of suppressing release of metallic ions or metallic oxides released from the structural materials and activated in a reactor core.
Piping, pumps, valves etc. used in a primary cooling water system of a nuclear power station are made of stainless steel, Stellite which is a Co based alloy, etc. (hereinafter briefly referred to as "structural members"). These metals are subject to corrosion or damages in the course of long-term services thereof. As a result, constituent metallic elements are dissolved in reactor cooling water (hereinafter briefly referred to as "cooling water") to be entrained into the reactor. Most of the dissolved metallic elements are converted into oxides thereof, which then adhere to fuel rods. In this state, the metallic elements are irradiated with neutrons. As a result, radioactive nuclides such as .sup.60 Co, .sup.58 Co, .sup.51 Cr, and .sup.54 Mn are formed. These radioactive nuclides are dissolved into primary cooling water again to suspend in the form of ions or insoluble solid components (hereinafter referred to as "crud"). Part of them is removed in a demineralizer, etc. for purification of reactor water, but the rest adheres to the surfaces of structural members in the course of circulation through the primary cooling water system. Therefore, the dose rate on the surfaces of the structural members increases, thus presenting a problem of radiation exposure of workers during the course of maintenance and inspection.
Accordingly, there have been proposed methods of suppressing causative dissolution of the above-mentioned metallic elements for decreasing the amount of the adhering radioactive substances. These methods include, for example, a method of suppressing corrosion of the structural members by using a corrosion-resistant material, and a method of suppressing corrosion of the structural members by introducing oxygen into a water supply system. However, in either method, corrosion of the structural members in the primary cooling water system including the water supply system cannot be sufficiently suppressed, and hence the amount of the radioactive substances in primary cooling water cannot be sufficiently decreased. Therefore, the does rate on the surfaces of the structural members due to the adherence of the radioactive substances thereto is increased.
On the other hand, methods of removing radioactive substances adhering to the structural members have been investigated and practiced. These methods include (1) mechanical washing, (2) electrolytic washing, and (3) chemical washing. The methods (1) and (2) encounter a difficulty in removing radioactive substances strongly adhering to the surfaces of the structural members, and are unable to decontaminate systematically over a wide area. The method (3) comprises dissolving an oxide film on the steel surface by a chemical reaction using a chemical such as an acid solution to remove radioactive substances present in the film. In this method, even if the dose rate is temporarily decreased, rapid recontamination occurs when the structural members are exposed to a liquid containing the radioactive substances dissolved therein at a high concentration again.
A method of suppressing adherence of radioactive substances by preliminarily providing an oxide film on the surfaces of structural members is disclosed in, for example, Japanese Patent Laid-Open Nos. 121197/1980 and 37498/1984. In this method, however, the adherence behavior of the radioactive substances markedly varies depending on the properties of the oxide film preliminarily provided. For example, the behavior of radioactive ions varies depending on the charged state of the oxide film. Also the rate of growth of oxide films newly formed on the surfaces of structural members after immersion in a liquid containing radioactive substances dissolved therein varies depending on the properties of the existent film. Thus a satisfactory film is not always formed.